support/config-fragments/autobuild: rename br-riscv64-musl config

For internal toolchains, we have a policy of naming the files with
"internal", to clearly distinguish them from external toolchain
configurations.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>